The invention relates to a method for determining the reducing agent concentration (NH3) in the exhaust-gas flow of an internal combustion engine.
In piston combustion engines, it is necessary, between the shaft end of the gas exchange valve on the one hand and the valve drive (cam of the camshaft, valve actuating lever, or the like) acting on it on the other, to dispose a valve play compensation element in order to compensate for temperature-caused changes in the length of the valve shaft and changes, caused by wear to the valve seat, in the height of the shaft end when the gas exchange valve is closed, relative to the valve drive. To that end, a hydraulic valve play compensation element is used, which essentially comprises a cup-shaped cylinder and a piston guided in it; the cylinder interior can be subjected to pressurized oil, so that the two parts can be spread apart and can each come into contact without play on the shaft end of the valve on the one hand and the valve drive on the other. Via a throttle restriction, which is for instance provided by means of a defined gap between the cylinder wall and the piston, it is also possible during operation to compensate for a change in the height of the shaft end relative to the valve drive, whether it is caused by thermal expansion or wear to the valve seat, since via the outflow of oil through the throttle restriction, the total length of the valve play compensation element can be shortened. Such hydraulic valve play compensation elements have proven themselves over time and today are used in practically all piston combustion engines.
The disadvantage of the hydraulic valve play compensation element, however, is that an oil supply must be provided especially for it, which necessitates considerable engineering and production effort and expense at the cylinder head.
A further disadvantage is that any change in the viscosity of the oil used definitively affects the function of such a hydraulic valve play compensation, so that it is practically impossible to design one optimal cam shape for all operating states. Another disadvantage is the high oil consumption, with the result that the oil pump must be designed even for critical operating states, such as idling while hot, and hence is designed to be oversized for normal operation.
Mechanical play compensating elements are also known from European Patent Disclosure EP-A 0 032 284, German Patent Disclosure DE-A 36 07 170, and International Patent Disclosure WO 90/10787.
The object of the invention is to overcome the disadvantages described by means of a mechanical valve play compensation element of simple design and high functional capability.
This object is attained in accordance with the invention by a mechanical valve play compensation element for a valve drive on a piston combustion engine, having a first pressure part, which is axially displaceable relative to a second pressure part and is held rotatably about the displacement axis, and having a torsion spring element, operative between the first pressure part and the second pressure part, that is axially resilient at least to a limited extent, and further having at least one helical surface on the first pressure part, with which surface a corresponding helical surface on the second pressure part is associated, the two forming a pair of helical surfaces, wherein the surfaces of the pair of helical surfaces being embodied as rough surfaces and being pressed against one another by the torsion spring element.
The advantage of this mechanical valve play compensation element is that in the state of repose, which is equivalent to the closing position of the gas exchange valve, as a result of the action of the torsion spring element, the two pressure parts are pressed apart to overcome any play, but contact one another with their helical surfaces. The valve drive can be formed directly by the cam of a camshaft, or via valve actuating levers (tilting levers, drag levers or the like). This assures that given the little force exerted between the two pressure parts during the closing time of the valve, any play that may be present is compensated for.
In the ensuing opening stroke, with the greater exertion of force for opening, the rough surface prevents the two pressure parts from rotating against one another, and thus prevents the compensation element from becoming shortened by being screwed together.
In one embodiment of the invention, the rough surface is embodied as a positive-engagement face, for instance in the form of a stair step profile with inclined step surfaces, so that only a compensation of an increasing valve play is possible, since the step edges each prevent reverse rotation of the pressure parts relative to one another in the direction of shortening the compensation element. The xe2x80x9cstep heightxe2x80x9d is expediently equivalent to an allowed working play.
A refinement contemplates a mechanical valve play compensation element in which a slide sleeve surrounding the first pressure part is provided, and a bracing spring element operative between the second pressure part and the slide sleeve is disposed, and furthermore on the first pressure part, a further parallel helical surface and a corresponding helical surface, offset in height from one another, are disposed on the slide sleeve and likewise form a pair of helical surfaces, the surfaces of the one pair of helical surfaces being embodied slidably and being pressed against one another by the torsion spring element, and at least one surface of the other pair of helical surfaces is embodied as a rough surface, whose surfaces are at a slight spacing from one another forming a working play and are each brought into contact with one another only during the valve opening event.
To initiate the valve opening, the elements contacting one another via the pair of helical surfaces, which as a rule are the first pressure part and the slide sleeve, are displaced in the direction of the second pressure part, counter to the exertion of force of the bracing spring, so that after a spacing forming a working play is bridged, the helical surfaces of the pair provided with rough surfaces come into contact with one another. The surface roughness of the two rough surfaces, upon touching one another, brings about a positive engagement, so that the two pressure parts, despite the actuation force acting in the opening direction, counter to the closing force of the valve spring, form an intrinsically rigid body, since rotation of the two pressure parts by becoming screwed into one another is not possible.
As soon as the closing position is regained, after the conclusion of the full valve stroke, the two pressure parts are pressed apart via the bracing spring, and via the exertion of force of the bracing spring between the two surfaces of the slidably embodied pair of helical surfaces, the two pressure parts are pressed apart, and by means of a relative rotation to one another, any valve play that may be present and is greater than the predetermined working play is compensated for. The slope of the pairs of helical surfaces extending parallel to one another is chosen such that no self-locking can occur in the pair of helical surfaces embodied slidably.
In an expedient feature of the invention, it is provided that the exertion of force of the bracing spring to the torsion spring element via the slide faces is markedly greater than the restoring force of the torsion spring element. This assures that changes in the valve play in both the positive and the negative direction, that is, spreading or contraction, caused by alternating operating states, for instance thermally, are assured as long as the rough surfaces do not touch another. As a result, even if the valve plays are changing, a greater play up to the predetermined, slight working play, will always be reliably compensated for. The depth of the roughness of the rough surface must be less than the allowed working play.
It is expedient if in a feature of the invention, ventilation bores are provided for the chambers that are surrounded by the pressure parts and/or by one pressure part and the slide sleeve. On the one hand, this prevents air cushions and/or accumulations of oil from being able to build up in these chambers, and on the other, this assures that by way of ventilation, however slight, oil mists can penetrate these chambers, thus lubricating the surfaces, moving relative to one another, of the individual parts.
Further characteristics and advantages of the invention can be learned from the description of the exemplary embodiments and the claims.